C100DBA Practice Test 4 — 10 Questions

Question 1

An application frequently queries a large collection by 'user_id' and 'status', sorting the results by 'timestamp'. Which index provides the most efficient execution plan?

Accepted Answer

{ user_id: 1, status: 1, timestamp: 1 }. To optimize this query, the ESR (Equality, Sort, Range) rule must be applied. The 'user_id' and 'status' fields are equality filters, while 'timestamp' is used for sorting. An index of {user_id: 1, status: 1, timestamp: 1} allows MongoDB to satisfy the query filters and provide the sort order directly from the index, avoiding a blocking sort operation in memory which is critical for performance on large collections.

Answer

{ timestamp: 1, user_id: 1, status: 1 }

Answer

{ user_id: 1, timestamp: 1, status: 1 }

Answer

{ user_id: 1, status: 1 }

Question 2

Which method should you use to update multiple documents in a collection and insert a new document if no matches are found?

Accepted Answer

db.collection.updateMany({filter}, {update}, {upsert: true}). The updateMany method modifies all documents matching the filter. When combined with the 'upsert' option set to true, it ensures that if no documents match the query criteria, a single new document is created containing the filter and update criteria. This pattern is critical for idempotent data ingestion pipelines where you must guarantee existence of records without performing separate check-then-insert logic, thus preventing race conditions.

Answer

db.collection.updateOne()

Answer

db.collection.replaceOne()

Answer

db.collection.save()

Question 3

Which read preference allows the application to read from the closest member of the replica set to minimize network latency?

Accepted Answer

nearest. The 'nearest' read preference is specifically designed to reduce latency by targeting the node with the lowest network round-trip time. This is highly effective for global deployments where applications are distributed across regions. Understanding read preferences is fundamental for application administration, as it allows developers to influence traffic distribution and resource utilization across the replica set, ensuring the application remains responsive even when the primary node is geographically distant from the client.

Answer

primaryPreferred

Answer

secondary

Answer

secondaryPreferred

Question 4

When performing a findOne operation, what does MongoDB return if no document matches the query criteria?

Accepted Answer

null. The findOne method is designed to return a single document or null. Returning null is the standard way for the driver and shell to indicate that the search criteria did not produce a match. This is important for application control flow, as developers must always verify that a document was actually retrieved before attempting to access its fields to avoid null pointer exceptions in their code.

Answer

An empty document ({})

Answer

An error code

Answer

An empty cursor

Question 5

What is the purpose of the 'OpLog' in a MongoDB Replica Set?

Accepted Answer

It provides a mechanism for asynchronous data replication.. The OpLog (Operations Log) is a capped collection that records all modifications to the data in the database. It is the core component that allows secondary nodes to replicate data from the primary node. By replaying the operations found in the OpLog, secondaries maintain an identical state to the primary. Understanding the OpLog is essential for diagnosing replication lag and ensuring the system is effectively synchronized across all nodes in the cluster.

Answer

It stores user credentials for database authentication.

Answer

It is used for storing index definitions and metadata.

Answer

It acts as a permanent, immutable backup of all data.

Question 6

Which member type in a MongoDB replica set is best suited for a geographically distant data center to provide read-only access without influencing elections?

Accepted Answer

Priority 0 Secondary. A priority 0 secondary is ideal for this scenario. By setting the priority to zero, the node is ineligible to become primary, preventing it from participating in elections. This is critical for nodes in remote locations where high latency could disrupt election timing or cause unnecessary re-elections, while still providing low-latency read access to users in that specific geographical region.

Answer

Arbiter

Answer

Hidden Member

Answer

Delayed Secondary

Question 7

What is the consequence of having a replica set with only two nodes and no arbiter?

Accepted Answer

The replica set will be unable to elect a primary if one node fails.. A replica set requires a majority of nodes to elect a primary. In a two-node set, a failure of one node makes it impossible to reach a majority, as one out of two is not greater than half. This design limitation is critical because it leads to the loss of a primary and the inability to elect a new one, resulting in a read-only database and complete outage for write operations.

Answer

The set will automatically double the voting power of the primary.

Answer

The replica set will function normally, but writes will be slow.

Answer

The primary will become read-only while the secondary remains idle.

Question 8

Which operator would you use to remove an element from an array field in a document?

Accepted Answer

$pull. The $pull operator is the primary tool for removing items from arrays based on a specified query condition. It is highly effective because it allows you to remove one or many matching elements in a single operation without needing to pull the entire document into application memory, modify the array, and push it back. This promotes efficient resource usage and data consistency.

Answer

$remove

Answer

$pop

Answer

$unset

Question 9

Refer to the exhibit. What is the primary function of the node with _id 2 in this replica set?

Accepted Answer

It provides a vote during elections to help maintain a majority.. An arbiter exists solely to participate in elections and break ties. It does not replicate data or serve read requests, making it a low-resource way to achieve a majority vote in a set. This is crucial for environments where a full-data secondary is cost-prohibitive but high availability is still required to maintain quorum during network partitions.

Answer

It stores a copy of the data and can serve read queries.

Answer

It performs automatic failover by hosting a redundant copy of the oplog.

Answer

It acts as a load balancer for incoming read requests.

Question 10

Refer to the exhibit. Which MongoDB philosophy is represented by this document structure?

Accepted Answer

Data locality for optimized retrieval patterns.. This document structure demonstrates the principle of embedding data. Instead of creating a separate collection for sensor readings and joining them via foreign keys, the readings are nested directly within the document. This approach minimizes the need for multi-table joins, which is a key tenet of MongoDB's design to keep data that is accessed together stored together, resulting in faster read performance and simplified application logic for time-series or hierarchical datasets.

Answer

Normalization for maximum data consistency.

Answer

Enforcement of a rigid schema using array types.

Answer

The use of an Oplog to track document changes.